1. Field of Invention
The present invention relates to a photosensitive composition suitable as a positive type image recording material and to a planographic printing plate using the photosensitive composition, and, particularly, to a photosensitive composition and a planographic printing plate using the same, the photosensitive composition being capable of forming a positive type image by using an infrared laser, the composition being suitable for a planographic printing plate for the so-called direct plate-making which enables writing by the heat of an infrared laser, thermal head or the like and can be directly performed based on digital signals from a computer or the like.
2. Description of the Related Art
Along with recent development of a solid laser and semiconductor laser emitting light within a range from the near infrared region to the infrared region, a system of effecting plate-making directly from digital data of a computer using these infrared lasers has been remarked.
A positive type planographic printing plate (material) for direct plate-making using an infrared laser is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 7-285275. This invention relates to an image recording material obtained by adding a material which absorbs light to generate heat and a positive type photosensitive compound such as a quinone diazide compound to a resin which is soluble in an aqueous alkali solution. The positive photosensitive compound functions as an dissolution-preventing agent, which substantially decreases the solubility of the resin soluble in an aqueous alkali solution, in an image portion. In a non-image portion, on the other hand, the photosensitive compound is decomposed by heat so that it does not develop dissolution-preventing ability, and can eventually be removed by development to thereby form an image.
It has been found as a result of the studies made by the inventors of the present invention that a positive image can be obtained even if these quinone azide compounds are not added to the image recording material. However, an image recording material from which these quinone azide compounds are simply excluded has the drawback that the stability of sensitivity to the density of a developer, namely, development latitude is impaired resultantly.
Meanwhile, onium salts and compounds which are insoluble in an alkali and can have hydrogen-hydrogen bonding are known to have a significant effect of preventing an alkali-soluble polymer from dissolving in an alkali. As an image recording material which is adapted to an infrared laser, compositions using a cationic infrared ray absorbing dye as an agent suppressing the dissolution of an aqueous alkali-solution-soluble polymer exhibit a positive effect as described in WO97/39894. This positive effect represents the effect of forming an image by making a polymer film of the laser-irradiated part lose the dissolution-suppressing ability, by making use of the heat generated when an infrared ray absorbing dye absorbs laser light.
Its image recording properties is sufficient on the surface of a photosensitive material irradiated with the laser. However, only insufficient image recording properties is obtained in the deep portion of the photosensitive material due to thermal diffusion. Therefore, it is hard to provide ON-OFF of exposed portions/unexposed portions in concerning alkali developing, posing the problem that a good image is not obtained (i.e., low sensitivity, narrow development latitude). The development latitude mentioned here indicates an allowable range in which a good image can be formed when the alkali concentration of an alkali developer is changed.